1. Field of the Invention
The present invention relates to a surface profile measuring instrument.
For example, the invention relates to a surface profile measuring instrument that detects a workpiece surface using a contact-type probe to measure the profile of the workpiece surface.
2. Description of Related Art
There has been known a measuring instrument that scans the workpiece surface to measure a surface texture and a three-dimensional profile of the workpiece such as a roughness tester, a contour measuring machine, a roundness measuring machine and a coordinate measuring machine.
For such a measuring instrument, a probe is used as a displacement sensor that detects the workpiece surface based on small displacements of a contact portion that is in contact with the workpiece surface (Document: JP-A-2004-61322).
FIG. 9 shows a profile measuring instrument 100 using a probe.
The profile measuring instrument 100 includes: a probe 200; and a three-dimensional drive mechanism 300 as a movement mechanism that moves the probe 200 in the three dimensions along a workpiece surface S.
As shown in FIG. 10, the probe 200 is a vibrating contact-type probe that includes: a stylus 211 having a contact portion 212 at a tip end thereof; a stylus holder 213 that supports the stylus 211; a vibrator 214 provided for the stylus holder 213, the vibrator constantly vibrating the stylus 211 at a natural frequency in an axial direction of the stylus 211; and a detector 217 that detects a change in the vibration of the stylus 211 to output a detection signal.
The vibrator 214 includes: a piezoelectric element 215 provided to the stylus holder 213 to vibrate the stylus 211; and a vibration circuit 216 that applies to the piezoelectric element 215 an output signal (such as a pulse and a sinusoidal wave signal) at a predetermined frequency.
The detector 217 includes: a piezoelectric element 218 that converts the vibration of the stylus 211 to voltage; and a detection circuit 219 that detects the voltage from the piezoelectric element 218 to output a detection signal.
The three-dimensional drive mechanism 300 is a three-dimensional drive mechanism that includes an X, Y and Z direction slide mechanism generally used in coordinate measuring machines.
Each of the axes of the three-dimensional drive mechanism 300 is provided with a linear encoder that detects a drive amount.
In the arrangement described above, when the contact portion 212 is moved along the workpiece surface S as shown in FIG. 11, the detection signal changes as shown in FIG. 12 (D) by the positional relationship between the contact portion 212 and the workpiece surface S. From a state where the contact portion 212 is free (FIG. 12 (A)), the contact portion 212 is brought into contact with the workpiece surface S (FIG. 12 (B)). When the contact portion 212 contacts to the workpiece surface S at a predetermined measuring force (FIG. 12 (C)), the vibration of the contact portion 212 is suppressed, so that the detection signal reaches a predetermined reference level.
The reference level is set in advance as a level obtained by subtracting from a detection signal value detected in the free state (the non-contacting state) of the contact portion 212 a change amount in signal generated when the contacting portion 212 is pressed at the predetermined measuring force.
Note that the measuring force is a force at which the contact portion 212 is pressed to the workpiece surface S when the contact portion 212 is brought into contact with the workpiece surface S for detection of the workpiece surface S.
The contact portion 212 is moved for scanning along the workpiece surface S while being pressed to the workpiece surface S such that the detection signal value is at the reference level. When the detection signal reaches the reference level, positional information of the probe 200 is sampled from slide amounts of the X, Y and Z axes of the three-dimensional drive mechanism 300. A contact point between the contact portion 212 and the workpiece surface S is calculated from the sampled position information of the probe 200, whereby the profile of the workpiece surface S is illustrated.
The detection signal is damped differently depending on an angle at which the contact portion 212 contacts the workpiece surface S.
Specifically, since the stylus 211 is vibrated in the axial direction thereof, the vibration of the stylus 211 is suppressed to different extents depending on whether the contact portion 212 abuts on the workpiece surface S in the axial direction of the stylus 211 or in a direction deviated from the axial direction of the stylus 211.
Accordingly, for the vibrating contact-type probe 200, the reference level of the detection signal is set on the premise that the contact portion 212 contacts the workpiece in the axial direction of the stylus 211.
Hence, in the case where the contact portion 212 contacts the workpiece surface S in the axial direction of the stylus 211, the vibrating contact-type probe 200 can be moved for scanning such that the detection signal becomes the reference level in order to scan the workpiece surface S at a constant measuring force.
However, depending on how the workpiece surface S is slant, the contact portion 212 may abut on the workpiece surface S not in the axial direction of the stylus but in a direction deviated from the axial direction of the stylus 211.
In this case, only a force component (in the axial direction of the stylus) out of the force acting from the workpiece surface S to the probe 200 can exert an influence on the detection signal change. Accordingly, the contact portion 212 is pressed too strongly to the workpiece in controlling the defect signal to the reference level, so that the measuring force cannot be maintained constant.
Thus, when the measuring force cannot be maintained constant, the workpiece surface S may be damaged due to the too strong pressing and the stylus 211 may bend, preventing an accurate detection of the workpiece surface S. Therefore, the workpiece of which workpiece surface S can be measured by the vibrating contact-type probe 200 has been limited to a workpiece having a substantially flat surface.